Steel Catenary Riser Fatigue Life Analysis On Riser-soil Interaction

As developments move into deeper water the understanding of structural performance of the riser can become critical to operational longevity.SCRs can be prone to fatigue damage,especially in the region where the riser pipe reaches the seabed-known as the 'touchdown zone'(TDZ).One of the key issues for SCR design is the assessment of fatigue damage due to repetitive loading over the lifetime of the riser.Accurate evaluation of the fatigue life of an SCR remains a major challenge due to uncertainty surrounding the interaction forces where the riser 'touches down' on the seabed.The results of a fatigue assessment depend significantly on the assumed pipe-soil interaction model at TDZ,which remains an area of uncertainty for designers.The pipe-soil interaction forces within the TDZ of an SCR strongly influence the fatigue damage that accumulates within the riser pipe.Structural analyses of SCRs usually consider only vertical pipe-soil forces and incorporate the pipe-soil interaction via linear springs.Fatigue life predictions for SCRs in the vicinity of the TDZ are heavily dependant on the assumed vertical stiffness between the riser and the seabed.For accurate fatigue life predictions to be made,a reliable evaluation of the seabed stiffness is required.Nonlinear models which incorporate tensile pipe-soil forces have been recently developed,P-y curve model and hysteretic seabed model,and indicate significantly decreased fatigue damage compared to the linear idealization.Firstly,based on the finite element method,the steel catenary riser fatigue estimate analysis method in time domain is provided,in which the SCR was analysed under the wave loads of the seastates in 8 directions respectively coupled with the corresponding 6 degrees of freedom movements of the floating,considering as the boundary condition in the top end of the SCR.The elastic model was used to simulate the interaction of riser-soil at the TDZ,and the SCR time domain nonlinear dynamic response was achieved under the loads of different occurrence probability seastates and motions of the floater.The time histories of stresses were given according to Von Mises combined stress expression to predict the riser fatigue life by use of S-N curve and Rainflow counting technique.The SCR fatigue damage results of 8 directions were added together with different probability of occurrence,and the whole SCR fatigue damage and fatigue life were achieved.The fatigue results at the top end and the TDZ was analyzed in different directions,and in different elastic element stiffness.Secondly,the P-y curve model was developed and combined with the Coulomb Friction'bilinear' model,which expresses the lateral resistance as the product of the effective submerged pipeline vertical force(submerged pipe weight minus hydrodynamic lift force)and a soil friction coefficient which depends solely on soil type.The conventional riser-soil design procedure is to model the interaction with spring links at intervals along the riser pipe at TDZ,these links provide a bilinear soil resistance in the lateral direction.The P-y curve model was developed in the 3-D space,utilizing the steel catenary riser fatigue estimate analysis method in time domain provided by the paper,the SCR time domain nonlinear dynamic response was achieved under the loads of different seastates and motions of the floater.The time histories of stresses were obtained to predict the riser fatigue life by use of S-N curve,and the whole SCR fatigue damage and motion fatigue life were achieved.Simultaneity,the hysteretic non-linear model was developed,the seabed is modeled using a hysteretic non-linear model proposed by Randolph in the vertical seabed direction,and SAFEBUCK soil models in the lateral seabed direction.The hysteretic non-linear was extended to the 3-D space,utilizing the steel catenary riser fatigue estimate analysis method in time domain provided by the paper,the SCR time domain nonlinear dynamic response was achieved under the loads of different seastates and motions of the floater.The time histories of stresses were obtained to predict the riser fatigue life by use of S-N curve,and the whole SCR fatigue damage and motion fatigue life were achieved.The results show that the TDZ response involves the degradation of the seabed soil stiffness due to cyclic loading.Furthermore,the more highly adopted approach and developed lateral SCR-soil interaction model,improves the dynamic response of the soil stiffness and riser penetration,which affects the global riser dynamic performance in the TDZ.Finally,a plasticity framework of riser-soil interaction model in clay soil seabed has been developed in this dissertation,and the fatigue life of an SCR was analysed integrating the linear springs model,P-y curve model,hysteretic seabed model and plasticity framework model respectively into a structural analysis program in the time domain.According to the comparisons of different models,the fatigue life analysis results of the plasticity framework are reasonable and the horizontal effects of pipe-soil interaction can be included.The use of a plasticity framework to encapsulate the behavior of a small section of the riser pipe at TDZ and the underlying soil offers an attractive alternate framework.By expressing the pipe-soil behavior purely in terms of the loads on the pipe and the corresponding displacements,a more fundamental understanding of the pipe-soil interaction under combined loading can be expressed in a terminology consistent with the pipe structural analysis.Integrating the plasticity framework model into a structural analysis program allows efficient modeling of the riser-soil interaction behavior,and the reasonable results can be achieved.In conclusion,applying advanced nonlinear SCR-seabed vertical and lateral interaction models that incorporate the SCR's cyclic motions using a finite element model can provide a realistic technique for predicting the dynamic response of the SCR in the TDZ.These models can be used to analyse soil resistance to SCR movements under hydrodynamic loading and determine the interaction's influence on the global riser structural dynamic behavior and the fatigue performance with better accuracy.